Angry customers have complained about not being able to speak to anyone at Telstra.

A Telstra spokesperson told news.com.au the telco did not know what caused the outage but they were investigating the problem.

"Telstra lost connectivity progressively to its international data network at around 1.40 this afternoon meaning customers could not access international websites from either their mobile or fixed services," a spokesperson said.

"Customers who couldn’t access international sites were then overloading domestic sites causing major issues in accessing Australian sites as well."

"The vast majority of customers have been progressively coming back on line since 2.25 this afternoon. Telstra’s engineers are continuing to monitor the network and identify the cause.

"We are also experiencing a high volume of calls to our customer service centres which means some customers may be having difficulties getting through at this time.

"Customers accessing our online services may also be experiencing difficulties.

"We will continue to keep our customers updated."